The use of a small diameter wire guide in the vessel of a patient such as the coronary vessels facilitates placement of a guiding catheter or another medical device thereover to the treatment site.
One flexible wire guide for use in cardiovascular procedures includes an elongated shaft with a coil spring mounted on the distal end of the shaft. The elongated shaft has a tapered portion near the distal end to gradually increase the flexibility of the distal end of the wire guide. The elongated shaft also includes a stepped down or significantly reduced diameter portion that is positioned proximal the tapered portion. The stepped down portion is typically formed by plunge grinding, which causes abrupt shoulders at the junctions with the main and tapered portions of the shaft. The coil spring is attached to the elongated shaft around the stepped down portion. The stepped down portion provides a recess for positioning the coil spring therearound and flowing solder therein to fixedly position the coil. In addition, the outer diameter of the wire guide about the proximal end of the coil spring remains constant for preventing a catheter from catching on the wire guide as the catheter is advanced thereover. A problem with this flexible wire guide is that the abrupt shoulder at the end of the stepped down portion of the shaft provides a stress point at which a fracture is prone to occur through the reduced diameter. This problem is compounded by the soldering of the coil spring to the stepped down portion, which heats or anneals the stepped down portion and further weakens the stepped down portion of the shaft. In use, the proximal end of the wire guide is rotated and advanced through the vessels of the vascular system, while the distal end bends and flexes. When stress is applied to the weakened portion of the elongated shaft, the shaft can fracture and break. As a result, the distal portion of the wire guide including the coil spring is deposited in the vascular system of the patient. This fragmented portion of the wire guide must be removed using a retrieval device or more invasive methods such as open surgery.
Another steerable wire guide includes a main core wire that is ground down near its distal end to form a shoulder and a reduced diameter, cylindrical distal region of increased, uniform flexibility. The wire guide also includes a coil that is welded to the shoulder region for facilitating a relatively smooth transition between the outer surface of the main core wire and the proximal end of the coil. A problem with the use of this wire guide is that the distal region has an abrupt change in flexibility rather than a continued increase in flexibility toward the distal end. If the distal region is too flexible, it deflects and will not track the vessel wall. If the distal region is too stiff, the risk of vessel perforation is significant. In addition, the distal end of the wire guide having an abrupt change in flexibility is easily dislodged from the target vessel and moves around as the catheter is advanced over the guide. Thus, the difficulty in inserting a catheter over the wire guide into a branching or target vessel is compounded and often thwarted with the dislodged guide. Another problem with the use of this wire guide is that the ground down shoulder has an abrupt, sharp radius. Again, the abrupt reduction in diameter weakens the main core wire at the point of attachment of the coil. The weakened point in the core wire is subjected to stress caused by the bending coil and, as a result, the wire guide can break at that point, depositing the distal region of the wire guide in the vessel of a patient, as previously described.